I have honestly never seen a 6 lb slab of baby backs! But, 3 3/4 hours, even with the hour in foil at 350 just doesn't sound long enough for a rack that size. I prefer baby backs that are between 2.5 and 3 lbs per rack and I cook them at 260-ish for 2.5 hours, then foil for 1.5 hours and then 15 minutes out of the foil to glaze. Your were twice as big, so I would have expect a longer cook time than mine, not shorter.
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